Intravascular probes which include ultrasound imaging crystals are well known. For example, it has previously been proposed to mount a piezoelectric crystal element (conventionally termed a "transducer") on or within a catheter of the type which can be inserted into a blood vessel. Once the probe has been inserted into a blood vessel, the transducer is electro-mechanically excited (as by the application of an electrical signal) to cause emission of ultrasonic energy into the surrounding tissue. While much of the emitted energy is absorbed by the surrounding tissue, a sufficient amount of energy is reflected back toward the transducer to permit imaging (with reflection occurring principally at the interfaces between different types of biological material, e.g., the interface between blood and the vascular wall, the interface between blood and lesions adhered to the vascular wall etcetera).
The transducer, in turn, produces weak electrical signals in response to electro-mechanical excitation by the returning reflected ("echo") ultrasonic energy. These weak electrical signals can be used to determine the geometry and/or other characteristics of the blood vessel, for example, to determine whether or not the blood vessel contains lesions or other abnormalities. These determinations are usually termed "imaging" since suitable video and/or other signal monitoring equipment are employed to convert the weak electrical signals produced by the transducer into human-readable form. Information gained from such imaging thus may assist the physician in a vascular treatment in real time or in diagnosing a patient's particular ailment or disease so that suitable therapy can be prescribed.
Intravascular imaging through 360.degree. has also been proposed. For example, in the above-referenced U.S. Pat. No. 4,841,977, novel intravascular ultrasonic imaging probes are disclosed having transducer arrays which include radially spaced-apart transducers. These radially spaced apart transducers thereby image corresponding radial segments of the vessel interior under examination (with conventional algorithms being utilized when necessary to "fill in" missing image segments through interpolation and/or partial images to provide sufficient information to a viewer).
It has also recently been proposed in U.S. Pat. No. 4,794,931 to Yock to provide intravascular imaging probes with a stationary transducer and an ultrasonic wave reflector which is rotatable and longitudinally movable relative to the transducer. (See, FIGS. 10 and 11 of Yock '931, and the corresponding description thereof). Moreover, it will be observed that the imaging devices disclosed in Yock '931 are each provided with a forwardly extending guide wire which serves to guide or steer the housing (which includes the transducer and reflection mirror) as the probe is introduced into the vessel of the patient's vascular system.
Miniaturization of ultrasonic imaging probes which are capable of providing real time images through 360.degree. presents several technical obstacles. For example, due to the miniature size of the components, it has been found that rotation of the transducer and/or any reflective mirror must be effected with virtually no "play" being present--otherwise skewed and/or inconsistent alignment of the transmitted and returned ultrasonic energy may result. Such imprecise and/or inconsistent ultrasonic energy alignment may therefore deleteriously affect the signal-to-noise ratio of the electrical signals produced by the transducer which, in turn, could result in imprecise and/or unrecognizable images.
While it might be envisioned that mechanical bearings could be provided to ensure virtual absolute coaxial rotation of the transducer and/or mirror, in reality, such bearings are too large and cumbersome to be used in ultrasonic imaging probes of sufficiently miniaturized size for insertion into equally small sized blood vessels of a patient's cardiovascular system, for example. Hence, the use of such bearings as has been proposed in prior large scale ultrasonic imaging devices conventionally termed "endoscopes" (such as those disclosed in, for example, U.S. Pat. Nos. 4,572,201; 4,466,444; 4,442,842; and 4,391,282), is entirely inadequate for use in miniaturized intravascular ultrasonic imaging devices of the type contemplated by the present invention.
It is also necessary that the ultrasonic imaging probe does not pose an unreasonable risk to the patient during use. For this reason, any rotational components should be mechanically isolated from the tissue in the patient's intravascular system--e.g., so as to prevent inadvertent and/or undesired tissue abrasion which might otherwise occur during operation if the rotational components were "exposed" to the patient's tissue. However, mechanically isolating these components creates further technical hurdles since the ultrasonic waves must not be attenuated by the isolating structure to an extent which would disrupt the obtained image.
It is towards attaining solutions to the above-noted problems that the present invention is directed. Broadly, the present invention is directed to an ultrasonic imaging probe of sufficiently miniaturized dimensions which enable the probe to be used within vessels of a patient's cardiovascular system, and which is capable of providing an image of such vessels through 360.degree. by rotation of a distally located transducer subassembly.
Important to the present invention, the transducer subassembly operates within a guide catheter previously positioned within the patient's intravascular system in a manner to be described in greater detail below. The guide catheter includes a distal section forming a "window" which minimally attenuates and/or reflects ultrasonic energy, and which mechanically isolates the rotational transducer subassembly from surrounding tissue. Perhaps equally significant, this distal region of the guide catheter serves the additional beneficial function of providing a distal bearing surface for the transducer subassembly and thereby ensures that virtual absolute coaxial rotation of the transducer subassembly relative to the guide catheter's axis will occur.
An essentially rigid tubular drive shaft is provided at the probe's proximal (and patient-external) end and is operatively connected to suitable motive means for imparting the desired rotation direction and velocity to the distally located transducer subassembly. In this regard, a torque cable interconnects the proximally located drive shaft and the distally located transducer subassembly so that the rotation direction and velocity of the former is transferred to the latter.
The torque cable employed in the probe of the present invention is formed of inner and outer subcables fabricated from wires which are helically wound in opposite directions relative to one another. Thus, the wires forming the inner and outer subcables are directionally wound such that their adjacent windings respectively tend to expand and contract radially when the subassembly is rotated in an intended rotational direction. This responsive expansion/contraction of the windings, in turn, effects an essentially rigid union between the inner and outer torque wires so that rotational movement provided by the motive means is reliably transferred to the transducer subassembly.
The preferred torque cable exhibits maximum strength under torsion, yet minimal strength under tension (i.e., exhibits minimal longitudinal stiffness). In order to increase the torque cable's longitudinal stiffness (i.e., increase its strength under tension), a suitable polymer material (e.g., low to medium density polyethylene) is impregnated into the interstices between the adjacent torque cable windings. Without such polymer impregnation, the torque cable may, during use, oscillate radially within the guide catheter and thereby cause the transducer subassembly to rotate at variable angular velocities. However, by increasing the longitudinal stiffness of the torque cable, essentially constant angular velocity may be transferred to the transducer subassembly since disturbing radial oscillations of the torque cable within the guide catheter will have been minimized (if not eliminated).
Additional longitudinal strengthening of the torque cable may be provided by means of at least one elongate strengthening element which is positioned within the torque cable, extends its entire longitudinal length, and is fixed to its proximal and distal ends. The strengthening element serves the beneficial function of longitudinally positionally fixing (tethering) the proximal and distal ends of the torque cable to thereby insure that it can easily be withdrawn from the patient without damage. And, in the unlikely event that the torque cable fails (e.g., severs) during use, the strengthening element will allow the entire torque cable to be withdrawn from the patient since it is connected to both the proximal and distal torque cable ends, thereby enhancing the safety of the probe assembly of this invention. Preferably, the strengthening element is in the form of at least one synthetic fiber monofilament.
In use, a conventional guide wire having a fluoroluminescent tip (e.g., formed of gold) will be inserted percutaneously into a desired vessel of a patient's vascular system, for example the patient's femoral artery, and is maneuvered so that its distal end is located in the particular vessel desired to be imaged. Progress of the guide wire can be visually followed by the attending physician using standard fluoroscopic techniques.
Next, the guide catheter is inserted telescopically over the now stationary guide wire. The guide catheter will preferably have a fluoroluminescent marking at its distal end so that the attending physician can similarly follow its progress over the guide wire using fluoroscopic imaging techniques. With the guide catheter properly positioned, the physician will then withdraw the guide wire so that the transducer subassembly can be inserted within the guide catheter's now vacant lumen.
It will be appreciated that the fluoroluminescent marking at the distal end of the guide catheter will provide a convenient stationary reference from which the physician can accurately position the transducer subassembly (which itself is entirely visible fluroroscopically). Thus, by operating the transducer subassembly, ultrasonic images corresponding to 360.degree. "slices" of the vessel will be obtained. And, these images can be obtained along a longitudinal section of the vessel since the transducer is both rotationally and longitudinally moveable within the guide catheter.
Once ultrasonic imaging has been completed and the desired information obtained, the physician may withdraw the transducer subassembly from the guide catheter and leave the guide catheter in position. Thus, the guide catheter may be employed as a channel for the introduction of suitable therapeutic probes (e.g., low profile angioplasty devices) and/or the delivery of pharmaceuticals to treat the afflicted site in the patient's cardiovascular system. Thus, the guide catheter will provide a convenient common path for both diagnosis (via the transducer subassembly of this invention) and treatment (e.g., via use of a separate therapeutic device and/or pharmaceuticals) of afflicted sites in a patient's cardiovascular system.
The probe and the imaging technique briefly described above represent a significant departure from what is believed to be the current wisdom in this art. For example, as evidenced by Yock '931, the conventional wisdom in this art is to provide an "all-purpose" intravascular imaging probe--i.e., one that includes integral guide wire structures so that the entire probe can be inserted directly into the patient's vascular vessels. In direct contrast, the present invention relies on a conventional discrete guide wire which serves as a means to position a guide catheter, the latter defining an adequately sized lumen to later accept (i.e., when the guide wire is withdrawn leaving the guide catheter in place) the transducer subassembly. In this manner, the imaging probe of this invention is especially well suited for use in the tortuous paths of very small sized coronary arteries and solves many of the problems briefly discussed above which have plagued conventional probes used in intravascular ultrasonic imaging.
These advantages, and others, will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiment.